Theory of Operation
1720/1721
4–21
The Error Amplifier regulates the +5 V output by feeding an error signal to the
Pulse Width Modulator. VR1 is a 2.5 V shunt regulator containing an op-amp
and a voltage reference. The +5 V is divided by R69 and R70 to provide 2.5 V
to VR1, with fine adjustment provided by R99. C40 and R71 determine the gain
and frequency response of VR1. VR4 controls overshoot of the +5 V at power
up. R98 and CR26 provide a minimum operating current for VR1. R68
decouples C39 from VR1. Overvoltage protection for the +5V supply is
provided by a crowbar circuit formed by Q11, VR3, R13, and R14.
The 80 kHz sawtooth waveform at U3 pin 3 trips comparator U3. U3 pin 1 then
feeds a trigger pulse to one-shot U4. U4 pin 13 outputs a 300 ns pulse to the 130
mA current source consisting of Q7 and Q8. When Q8 turns on, T2 pin 2 is
pulled down until CR15 (Error Amplifier) is forward biased. The negative going
pulse at T2 pin 2 is peak detected by CR16 and C46. The dc voltage present at
the anode of CR16 feeds the pulse width modulator and the Output HV
Shutdown circuit. CR29 resets T2 between pulses.
If the +5 V is below 4.9 V, the Error Amplifier will cause the Peak Detector
output to go below 2.9 V. The output of comparator U3B will pull low and
shutdown pulse width modulator U5. C47 and R96 delay the operation of U3B
long enough for the power supply to power up. If the +5 V does not reach 4.9 V
within 50 ms of power up, U3B will shut down the switcher. The power supply
will then cycle on and off every couple of seconds
.
Over voltage protection is provided on the +5 V output by a crowbar circuit
composed of Q1, VR3, R13, and R14. If the +5 V output exceeds approximately
+5.5 V, VR3 will start to conduct. When VR3 is drawing enough current through
R13 to raise SCR Q11 gate voltage above its cathode by approximately 0.7 V,
Q11 will turn on. This shorts the +5 V output to ground, forcing the primary
circuit into current limit.
Error Amplifier
Feedback
Transformer Driver
and Peak Detector
Output HV Shutdown
Over Voltage Protection
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